The invention relates to a method for producing internal connections between two terminal equipments installed in a ring or bus structure at one and the same subscriber of an integrated services digital network with a basic access of the subscriber to the exchange via a two-wire central office line. Two digital base channels having the same bit rate and a control channel having a bit rate lower in comparison thereto are provided in a connection between the subscriber terminal and the exchange, situated in a holding condition or state.
In an integrated services digital network of the above type, also refered to as ISDN, different telecommunications services which relate to voice, data, text, facsimile, still picture communication, and remote control information are combined into a uniform digital communications network.
In contrast to previous telecommunications networks, an ISDN subscriber has more than one terminal equipment and can simultaneously execute a plurality of activities with the network.
The execution of these activities occurs via a subscriber line circuit leading to the exchange on which two base channels, so-called B-channels witha bit rate of 64 K bits/s, are formed.
Also formed on this subscriber line circuit, particularly for the transmission of the control characters, is a control channel or D-channel, whose bit rate amounts to 16 K bits/s in the actual case. In addition to exchanging control characters, this D-channel, however, can also be used for the transmission of telemetry data, and packet-switched data.
There is also interest in being able to produce a connection between two terminal equipments of one and the same subscriber, i.e., an internal connection, for example in the case of a boss/secretary system. This is true when, in the course of a connection to terminal equipment of a different subscriber existing via the exchange, an inquiry is to be made, for example, from the secretary station to the boss station. Such internal connections, of course, can also be of interest between other arbitrary terminal equipment units of a subscriber.
However, there is no intent to lend the subscriber connection the properties of a private branch exchange wherein internal connections are also possible without the existence of a connection to the exchange.